Method of manufacturing strong black heart malleable cast iron



Patented Oct. 11, 1932 UNITED STATES PATENT OFFICE TABIO KIKUTA, OITOIBATA, I'UKUOKA KEN, JAPAN, ASSIGNOR TO TOBATA IMONO KABUSHIKI KAISHA,OF TOKYO, JAPAN, A CORPORATION OF JAPAN METHOD OI MANUFACTURING STRONGBLACK HEART MALLEABLE CAST IRON R0 Drawing. Application filed November27, 1929, Serial No. 410,255, and in Japan September 1 0, 1929.

in iron oxide or other oxidizing agents of 4 to 15% by weight of thecharge of the white iron castings in the pot, accomplishingde-carbonization of the white cast iron while it is held at a hightemperature in parallel to the graphitization. The object thereof is toobtain black heart malleable cast iron as strong as cast steel bycarrying out the operatlon more economically and m shorter time than bythe known method and at the same tim'eby effecting the de-carbonizationperfectly.

This invention resembles the method of graphitizing in two stages thecombined carbon contained in white cast iron without employin packingmaterial as described in my prior Patent No. 1,692,842, but diifers ineffecting the decarbonization by aid of iron I v oxide or otheroxidizing agents put in the annealing pot amounting to from 4 to 15% ofthe charge of the castings as mentioned above. According to the knownannealing method using packing material, sand, iron oxide, slag, etc.are usually used as packing material and the space left in the annealingpot after it is charged with white iron castings is almost completelyfilled with such material so that when the castings have irregular ormechanical shapes as is quite often the case in malleable cast iron, thequantity of these packing materials will become considerably large andconsequently the longer time will be required for annealing. Theprincipal object of em loying packing material in such a metho is toutilize the heat absorbed by it and cool slowly at a temperature rangejust below Ar transformation point, so that pearlitic cementite producedat Ar transformation point of iron after the graphitization of freecementite can be graphitized. The de-oarbonization which takes place inusing iron oxide packing in said known method and for reducingdeformation of castings caused during the annealing due to the use ofpack ing material is only a subsidiary action, and is notthe main objectof the annealing oper ation. This is the reason why sand or slag isemployed more often than oxide in manufacturing black heart malleablecast iron. In the manufacture of white heart malleable cast iron, thede-carbonization being the primary object, it is the well known factthat iron oxide is employed as packing material.

However, when packing material is used, it takes considerable time toheat the charge of castings in the 'pot uniformly, because the packingmaterial has big heat absorbing capacity in itself and further it is apoor conductor of heat. Consequently, it requires a larger quantity offuel to anneal, increases deterioration of the annealing pot and it hasanother disadvantage in that charging castings to or their dischargingfrom the pot is naturally slow as compared with the method in which nopacking material is used.

The annealing method in which white cast iron is graphitized in twostages without packing material is however accompanied by more or lessdecarbonization, but as the decarbonization is effected mainly by oxygencontained in the air present in the space inside the pot, its amount isvery small. When black heart malleable cast iron is viewed from themctallographical point, temper carbon is distributed in globular form onthe ground of silico-ferrite and the tensile strength of black heartmalleable cast iron is controlled by the quantity of this carbon.

Therefore, if'the content of this free carbon is reduced as much aspossible, it is possible i from the iron-carbon diagram, will beextended and as a result increases liquid shrinkage, thus increasing thetendency of producmg porous or rough parts 111 the interior of thecasting and makmg 1t diflicult to obtain sound castings. For thisreason, it is prefer able to restrict the carbon content in white castiron to about 2.5%, and it is not advantageous from the foundry Viewpoint to make it below 2.2%. Therefore, the most advan- 4 tageous methodof obtaining strong and pertion.

feet black heart malleable cast iron is to maintain the carbon contentof white cast iron at nearly 2.5% and make the subsequent annealingaccompanied by the de-carbo niza- However, for the purpose of effectingsaid de-carbonization, it is not advantageous to fill the annealing potwith such packing material as iron oxide, because it increase .cost ofannealing.

After careful'research and many experiments, I have 'found that theobject of the de-calbonization can be attained perfectly by adding tothe charge in the pot 4 to 15% by weight of iron oxide, and I find it ispreferable to effect the de-carbonization in this manner rather than bypassing oxygen gas or air during the annealing operation through thecastings, as the latter method produces-an unfavorable result due to theexcessive oxidation ofthe iron itself. It may be added that theoxidation of the iron can be almost prevented by employing the properamount of a suitable oxidizing agent like iron oxide. 1

The following are examples of carrying this invention intopractice byusing two series of white cast iron having the following chemicalconstituents A small electric annealing furnace having -a capacity ofabout 200 kilograms has been used.

" EXAMPLE I (With no de-carbonizing agent) Amount of white iron castingscharged (includmg test pieces), 108 kilograms.

Hours required for graphitization of the first stage, 29 at 925980 C.

Hours required for graphitization of the second stage, 49 at 710 C.

The result of the annealing was good, showing the following tensilestrength and bending angle Content of carbon (per I Bending cent)Tension test test Numheirof sump-e Tensile Elongamamml 3351Difierstrength tion in Bending in in ence (kg.] 2 (per angle g g mm.)cent) No. 1 2. 58 2. 52 0. (16 33. 62 12.0 150 No. 2 v 2.60 2. 39 0.2133.00 13.0 100 From this it will be seenthat the amount ofde-carbonization during the annealing 1s only about 0.13% on an average.

EXAMPLE II (With dc-carbonizing agent) Content of carbon (per Bendingcent) 7 Tension test test Numb?- of Samp-e Tensile Elongamatenal 523 tgDifiers'tz'lgnglth $921 in Bendling erence g. per am; 6 mg mg mm.) cent)No. 1 2. 58 1.90 0. 68 36. 31 17. 0 330 N0. 2.-..-- 2. 60 1. 98 0. 6237. 01 18. 0 360 The amount of de-carbonization is O.65% on an average.The tensile strength and elongation are far greater than when node-carbonizing agent was employed as in Example I.

EXAMPLE III (With de-carbonizing agent) Amount of white iron castingscharged (including test pieces), 118 kilograms.

Hours for, graphitization of the first stage, 24 at 925980 0.

Hours for graphitization of the second stage, 49 at 710 C.

De-carbonizing agent, (same hematite for Example II was repeatedlyused), 6.4 kilograms.

The annealing was satisfactory and the following results were obtainedThus, it will be seen that the average amount of de-carbonization isabout 0.88% and the tensile strength and elongation are furtherimproved. Hematite powder used ,as de-carbonizing agent has beenreduced, and the amount of the reduced 11011 was As regards thesephenomena of decarbonization, I have measured each of them numericallyand thus have not only made the mechanism of-the de-carbonization clear,but also was able to determine the amount of the de-carbonizing agentrequired to obtain the necessary amount of de-carbonization of castin s.That is to say, the de-carbonization ta es place chiefly owing to thecatalytic action of carbon dioxide gas (CO and the carbon contained inwhite cast iron is not only removed, but at the same time the oxygencontained in iron oxide is lost. Thus, the de-carbonization of whitecast iron is accompanied by the reduction of iron oxide. Therefore, thepresent method also serves to reduce iron sand.

I claim:

1. Method of manufacturing strong black heart malleable cast ironconsisting in charging an annealing pot with white iron cast-' ings andwith 4 to 15% byweight of iron oxide, maintaining the same at atemperature between 900 C. and 980 C. for a period of time varying fromtwenty to fifty hours so as to graphitize the free cementite containedtherein, then lowering the temperature to between 730 C. and 650 C. andmaintaining that temperature for a period of time varying from ten tofifty hours thus graphitizing pearlitic cementite and efi'ectingdecarboniz ation of the white cast iron.

2. Method of manufacturing strong black heart malleable cast'ironconsisting in subjecting a charge of white iron castings t0- getherwithiron oxide to the amount of 4 to 15% by weight of the white ironcastings to a temperature between 900 C. and 980 C. for a period of timevarying from twenty to fifty hours, then lowering the temperature to apoint between 7 30 C. and 650 C. and maintaining that temperature forfrom a period of time varying ten to fifty hours.

In testimony whereof I have aflixed my signature.

TARIO KIKUTA.

